On 13.12.2017 04:10, Wanpeng Li wrote: > From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> > > Reported by syzkaller: > > WARNING: CPU: 0 PID: 12927 at arch/x86/kernel/traps.c:780 do_debug+0x222/0x250 > CPU: 0 PID: 12927 Comm: syz-executor Tainted: G OE 4.15.0-rc2+ #16 > RIP: 0010:do_debug+0x222/0x250 > Call Trace: > <#DB> > debug+0x3e/0x70 > RIP: 0010:copy_user_enhanced_fast_string+0x10/0x20 > </#DB> > _copy_from_user+0x5b/0x90 > SyS_timer_create+0x33/0x80 > entry_SYSCALL_64_fastpath+0x23/0x9a > > The syzkaller will mmap a buffer which is also the struct sigevent parameter of > timer_create(), it will also call perf_event_open() to set a BP for the buffer, > so when the implementation of timer_create() in kernel tries to get the struct > sigevent parameter by copy_from_user(), rep movsb triggers the BP. The syzkaller > testcase also sets the debug registers for the guest, however, the kvm just > restores host debug registers when we have active breakpoints. I can observe > the dr6 single step bit is set and !hw_breakpoint_active() sporadically by print > when running the testcase heavy multithreading. The do_debug() which is triggered > by rep movsb will splash when (dr6 & DR_STEP && !user_mode(regs)). > > This patch fixes it by restoring host dr6 in sched_out if no breakpoint is active. > > Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> > Cc: David Hildenbrand <david@xxxxxxxxxx> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> > --- > v1 -> v2: > * move to sched_out path > > arch/x86/kvm/x86.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 1c5c7a3..76886c4 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -2964,6 +2964,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) > pagefault_enable(); > kvm_x86_ops->vcpu_put(vcpu); > vcpu->arch.last_host_tsc = rdtsc(); Can you add a comment like /* With active breakpoints we already restored all debugregs in vcpu_enter_guest(), however without active breakpoints we have to restore debugreg 6 before scheduled out. */ > + if (!hw_breakpoint_active()) > + set_debugreg(current->thread.debugreg6, 6); > } > > static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu, > Having the restore at the old place was for me easier to understands. But this moves it out of the hot loop. -- Thanks, David / dhildenb
